1. Field of the Invention
This invention generally relates to photolithography, and more specifically, the invention relates to methods and systems for determining grid offsets for a photolithographic tool.
2. Background Art
Step-and-expose and step-and-scan photolithography tools (referred to as “steppers”) are used extensively in the manufacture of large scale integrated circuits. A principal advantage of the use of steppers for the manufacture of integrated circuits is the ability of steppers to rapidly produce very fine patterns on the resist coated wafers.
In use, the stepper patterns wafers by producing a plurality of essentially identical fields that are placed adjacent to each other. The area of the wafer to be patterned is placed under the objective lens of the stepper and the field is exposed. The stepper then moves the wafer stage to the next portion of the wafer to be exposed. This process continues until all areas of the wafer requiring patterning have been exposed.
As integrated circuits require many levels of layers that must be properly placed relative to one another, steppers include an alignment system to aid in achieving the proper relative placement of the current level to the prior level or levels. However, the very first level to be printed on the wafer does not have access to alignment marks and this first level is exposed without the use of the alignment system. This first level exposure process without the aid of the alignment system is sometimes referred to as “blind stepping.” The first level exposure of multiple copies of identical fields can be modified by standard stepper systematic parameters. These include the field systematic parameters of field x magnification, field y magnification, field rotation, and field orthogonality. The first level exposure includes grid parameters of grid x magnification, grid y magnification, grid rotation and grid orthogonality.
In order to insure optimum overlay for subsequent levels, it is critical that the first level be exposed with known placement characteristics. To achieve these optimum results, it is essential that the field systematic parameters match the associated grid parameters. Also, when the stepper parameters that are applied at run-time are determined using feedback from lots previously exposed, it is important that the first level placement characteristics be consistent from lot to lot and preferably match the stepper baseline characteristics.